This invention relates to captive, internally threaded fasteners, more specifically to a clinch nut produced completely by means of a progressive stamping process. The invention is stamped and formed from flat coil stock into a fully functional internally threaded clinch nut that may be supplied in bulk form or on a reeled carrier strip.
Progressively-formed threaded inserts, an example of which is shown in U.S. Pat. No. 5,927,920, are known. While this method of forming a threaded insert is suitable for being fully inserted into a plastic sheet, it cannot be attached to a metal panel. Other types of threaded fasteners pressed into metal panels may be joined by a process known as xe2x80x9cclinchingxe2x80x9d whereby a cold flow of metal created during the forceful insertion process enters an undercut groove at the base of the nut. Heretofore a progressively-formed threaded part having clinch-attachment capabilities is unknown.
Such a device would be desirable since prior art clinch nuts are manufactured from a bar stock machining process or a wire forming process often requiring a costly secondary tapping step. Bar stock machining processes typically yield 1,000 pieces per hour with 70% scrap while wire-forming processes typically yield 13,000 pieces per hour with 15% scrap. There is therefore a need in the art for a clinch nut which may be inexpensively produced efficiently and with less scrap in the manufacturing process.
In order to meet the needs in the art described above, the present progressively-formed clinch nut invention has been devised. A three-sided progressively-formed threaded fastener is provided with wedge-shaped protrusions and angled side edges of shank tabs that extend from the base of the fastener. The protrusions displace sheet material into undercuts formed between the shank tabs and the base of the fastener. These structures provide the clinch attachment means when the fastener is pressed into a sheet. Except for the inclusion of these clinch features, the fastener of the present invention and its method of production is similar to that disclosed in the above-mentioned U.S. Pat. No. 5,927,920 which is included herein by reference as though fully set forth.
More specifically, the applicants have invented a threaded metal fastener comprising a tube-like channel having a polygonal cross-section and being comprised of a plurality of planar longitudinal faces intersecting at corner areas. A plurality of shank tabs extend downwardly from the bottom of each of the faces at the base of the channel. A plurality of threaded regions comprising longitudinal rows of indentations lie on the inside surface of each of the planar faces. A plurality of wedge-shaped protrusions extend downward from the corners. This forms a clinch profile at the bottom of the channel so that the fastener can be permanently affixed to a sheet of metal by clinch fit. The fastener has a triangular cross-section and is a folded strip of material with its first and second side edges meeting in abutment along a longitudinal seam. The first side edge includes a cutout and the second side edge includes a corresponding interlocking clasp tab. The seam is held together by inserting the interlocking side tab into the cutout.
The method of forming the present invention comprises providing an elongate strip of metal, cutting it into fastener blanks with arcuate indentations in a front surface with the indentations arranged in spaced longitudinal rows. The blanks each include shank tabs extending downwardly from a bottom edge. The method further includes folding the strip into a tube-like channel along longitudinal foldlines located between the rows of indentations such that wells of said indentations define chordal sections of a space corresponding to an externally-threaded member. A first edge of the blank includes a clasp, and an opposite edge of the blank includes a cutout corresponding to the shape of the tab. The clasp is inserted into the cutout to hold the edges in a fully-folded condition with the edges being in abutment along a longitudinal seam. The fastener blanks can remain connected to a common carrier portion of the strip after formation.
Not only can the present device be produced more efficiently and less expensively than a machined clinch nut, it provides many other advantages. For example, the current invention requires a smaller mounting hole when compared to the prior art resulting in closer center-line-to-edge distances and less panel distortion. Over-tightening of the mating screw in the clinch nut will not damage the threads, since the side walls flex outwardly and disengage to relieve excess torsional load on the threads prior to thread damage. Thus, neither the screw nor the clinch nut threads are damaged. Also, the present invention provides the key features of the ability to be produced in high volumes at high production rates with minimal scrap since it can be produced at rates of 21,000 pieces per hour with 15% scrap and no secondary tapping process. A portion of the scrap can be used as a carrier strip to facilitate automated installation eliminating the need for an expensive vibratory feeding bowl. Hence, a clinch nut progressively formed by die stamping as described herein can be made much less expensively. Other objects and advantages of the present invention will be apparent from the following drawings and description of the preferred embodiment.